Multi-color electro-optic displays

ABSTRACT

An electro-optic display comprising at least two separate layers of electro-optic material, with one of these layers being capable of displaying at least one optical state which cannot be displayed by the other layer. The display is driven by a single set of electrodes between which both layers are sandwiched, the two layers being controllable at least partially independently of one another. Another form of the invention uses three different types of particles within a single electrophoretic layer, with the three types of particles being arranged to shutter independently of one another.

REFERENCE TO RELATED APPLICATION

This application claims benefit of copending application Ser. No.13/113,567, filed May 23, 2011. The entire disclosure of thisapplication is herein incorporated by reference.

BACKGROUND OF INVENTION

The present invention relates to multi-color electro-optic media and todisplays incorporating such media.

The term “electro-optic”, as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. The optical property is typically color perceptible to thehuman eye.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the E Inkpatents and published applications referred to below describeelectrophoretic displays in which the extreme states are white and deepblue, so that an intermediate “gray state” would actually be pale blue.The terms “black” and “white” may be used hereinafter to refer to thetwo extreme optical states of a display, and should be understood asnormally including extreme optical states which are not strictly blackand white, for example the aforementioned white and dark blue states.The term “monochrome” may be used hereinafter to denote a drive schemewhich only drives pixels to their two extreme optical states with nointervening gray states.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin U.S. Pat. No. 7,170,670 that some particle-based electrophoreticdisplays capable of gray scale are stable not only in their extremeblack and white states but also in their intermediate gray states, andthe same is true of some other types of electro-optic displays. Thistype of display is properly called “multi-stable” rather than bistable,although for convenience the term “bistable” may be used herein to coverboth bistable and multi-stable displays.

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedby applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14 (11),845. Nanochromic films of this type are also described, for example, inU.S. Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of mediumis also typically bistable.

Another type of electro-optic display is an electro-wetting displaydeveloped by Philips and described in Hayes, R. A., et al., “Video-SpeedElectronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003).It is shown in U.S. Pat. No. 7,420,549 that such electro-wettingdisplays can be made bistable.

Particle-based electrophoretic displays, in which a plurality of chargedparticles move through a fluid under the influence of an electric field,have been the subject of intense research and development for a numberof years. Electrophoretic displays can have attributes of goodbrightness and contrast, wide viewing angles, state bistability, and lowpower consumption when compared with liquid crystal displays.Nevertheless, problems with the long-term image quality of thesedisplays have prevented their widespread usage. For example, particlesthat make up electrophoretic displays tend to settle, resulting ininadequate service-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat.Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic mediaappear to be susceptible to the same types of problems due to particlesettling as liquid-based electrophoretic media, when the media are usedin an orientation which permits such settling, for example in a signwhere the medium is disposed in a vertical plane. Indeed, particlesettling appears to be a more serious problem in gas-basedelectrophoretic media than in liquid-based ones, since the lowerviscosity of gaseous suspending fluids as compared with liquid onesallows more rapid settling of the electrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporationdescribe various technologies used in encapsulated electrophoretic andother electro-optic media. Such encapsulated media comprise numeroussmall capsules, each of which itself comprises an internal phasecontaining electrophoretically-mobile particles in a fluid medium, and acapsule wall surrounding the internal phase. Typically, the capsules arethemselves held within a polymeric binder to form a coherent layerpositioned between two electrodes. The technologies described in thethese patents and applications include:

(a) Electrophoretic particles, fluids and fluid additives; see forexample U.S. Pat. No. 7,002,728; and 7,679,814;

(b) Capsules, binders and encapsulation processes; see for example U.S.Pat. No. 6,922,276 and; 7,411,719;

(c) Films and sub-assemblies containing electro-optic materials; see forexample U.S. Pat. No. 6,982,178; and 7,839,564;

(d) Backplanes, adhesive layers and other auxiliary layers and methodsused in displays; see for example U.S. Pat. No. 7,116,318; and7,535,624;

(e) Color formation and color adjustment; see for example U.S. Pat. Nos.6,017,584; 6,664,944; 6,864,875; 7,075,502; 7,167,155; 7,667,684; and7,791,789; and U.S. Patent Applications Publication Nos. 2004/0263947;2007/0109219; 2007/0223079; 2008/0023332; 2008/0043318; 2008/0048970;2008/0211764; 2009/0004442; 2009/0225398; 2009/0237776; 2010/0103502;2010/0156780; and 2010/0225995;

(f) Methods for driving displays; see for example U.S. Pat. No.7,012,600; and 7,453,445;

(g) Applications of displays; see for example U.S. Pat. No. 7,312,784;and U.S. Patent Application Publication No. 2006/0279527.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called polymer-dispersed electrophoretic display, inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,U.S. Pat. No. 6,866,760. Accordingly, for purposes of the presentapplication, such polymer-dispersed electrophoretic media are regardedas sub-species of encapsulated electrophoretic media.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated withinmicrocapsules but instead are retained within a plurality of cavitiesformed within a carrier medium, typically a polymeric film. See, forexample, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to SipixImaging, Inc. Hereinafter, the term “microcavity electrophoreticdisplay” may be used to cover both encapsulated (includingpolymer-dispersed) and microcell electrophoretic displays.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, U.S. Pat. Nos.5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and6,184,856. Dielectrophoretic displays, which are similar toelectrophoretic displays but rely upon variations in electric fieldstrength, can operate in a similar mode; see U.S. Pat. No. 4,418,346.Electrophoretic media operating in shutter mode may be useful inmulti-layer structures for full color displays; in such structures, atleast one layer adjacent the viewing surface of the display operates inshutter mode to expose or conceal a second layer more distant from theviewing surface.

As already indicated, an encapsulated or microcell electrophoreticdisplay typically does not suffer from the clustering and settlingfailure mode of traditional electrophoretic devices and provides furtheradvantages, such as the ability to print or coat the display on a widevariety of flexible and rigid substrates. (Use of the word “printing” isintended to include all forms of printing and coating, including, butwithout limitation: pre-metered coatings such as patch die coating, slotor extrusion coating, slide or cascade coating, curtain coating; rollcoating such as knife over roll coating, forward and reverse rollcoating; gravure coating; dip coating; spray coating; meniscus coating;spin coating; brush coating; air knife coating; silk screen printingprocesses; electrostatic printing processes; thermal printing processes;ink jet printing processes; electrophoretic deposition (See U.S. Pat.No. 7,339,715); and other similar techniques). Thus, the resultingdisplay can be flexible. Further, because the display medium can beprinted (using a variety of methods), the display itself can be madeinexpensively.

There is today an increasing demand for color in all displays. Usersfamiliar with color televisions, color computer displays and colordisplays on cellular telephones and other portable electronic devicesmay regard monochrome display as lacking something in visual appeal evenin applications such as electronic book readers, where the display aimsto reproduce the look of a printed book, most of which are still printedin monochrome.

In conventional printing, full color images of high quality are formedby providing sub-images in each of three subtractive primary colors,typically cyan, magenta and yellow (“CMY”) (black may be included as afour primary in a “CMYK” system) that are overlaid (i.e., more than onecolor can be present at any point on the page) in such a way that lightis filtered through each sub-image before being reflected back from theunderlying white paper to the viewer. (Thus, a so-called “four color”,CMYK system is in reality a five-color system; the white color of theunderlying paper is part of the color formation system, as is readilyappreciated from the fact that this white color appears where no inkwhatever is present). In this arrangement of three or four overlaidsub-images, no area of the printed paper absorbs light unnecessarily,and thus an image of maximum brightness is obtained.

Prior art electrophoretic and similar electro-optic displays havetypically relied upon the use of reflective (light-scattering) pigments.Since no substantial amount of light passes through a layer of suchpigment, it is not possible to overlay sub-images of differing colors,and in a color display it is necessary to resort to “color area sharing”to render a palette of colors. For example, multiple differentsub-regions of the display may be provided with electrophoretic mediacapable of displaying different colors, for example red, green and blue.(Note that since there is no overlaying of sub-images of differentcolors, this type of display typically uses additive primaries ratherthan subtractive primaries). Alternatively, a monochrome medium can beused and a color filter array provided so that specific pixels canreflect specific primary colors. Either approach suffers, however, fromthe problem that only a fraction of the area of the display is availablefor reflection of each primary color, which adversely affects thebrightness of the image available. Hence, to improve the brightness of acolor reflective display, it is desirable to provide a display which candisplay any desired color at any pixel of the display, and thereby tomaximize the amount of light reflected to a viewer.

Multilayer, stacked electro-optic displays using are known in the priorart. In such displays, ambient light passes through sub-images in eachof the three subtractive primary colors, in a manner analogous toconventional color printing. U.S. Pat. No. 6,727,873 describes a stackedelectrophoretic display in which three layers of switchable cells areplaced over a reflective background. Similar displays are known in whichpigments are moved laterally; see for example International ApplicationPublication No. WO 2008/065605) or, pigments in microcavities are movedusing a combination of vertical and lateral motion. For a review of suchdisplays, see J. Heikenfeld, P., et al., Journal of the SID, 19(2),2011, pp. 129-156. In these prior art displays, each pixel of each layermust be capable of being driven independently so as to concentrate ordisperse the pigment particles on a pixel-by-pixel basis. This requiresthree separate pair of electrodes, each of which typically comprises anactive matrix backplane having a matrix of thin-film transistors, and anopposed continuous counter-electrode. Two of the active matrixbackplanes must be as transparent as possible, as must be eachcounter-electrode. This approach suffers from the severe disadvantagesof the high cost of manufacturing such a complex arrangement ofelectrodes, and from the fact that in the present state of the art it isdifficult to provide an adequately transparent backplane, especially asthe white state of the display requires that light pass through severallayers of such transparent electrodes; in practice, the light losses inthe electrodes have a severe adverse effect on the brightness of theimage produced by the display.

Those skilled in the imaging art know that it is necessary to provideindependent addressing of each primary color in order to render afull-color image. This is illustrated graphically in FIG. 1 of theaccompanying drawings, which shows a “color cube” in which the verticescorrespond to white, the three subtractive primary colors (yellow,magenta, and cyan), the three additive primary colors (red, green andblue) and black. As shown by the arrows, any point inside or on thesurface of the color cube can be defined by three (orthogonal)co-ordinates, namely, the distances along the white-yellow axis, thewhite-magenta axis, and the white-cyan axis. These distances correspondto different optical densities in the subtractive primary colors,ranging from zero (i.e., white) to about 2 (corresponding 99% absorptionof light of the corresponding additive primary spectral range). Thenumber of discretely addressed independent states required to render thefull color gamut of a display is the number of yellow states plus thenumber of magenta states plus the number of cyan states. The number ofcolors that can be rendered, however, is the product of these threenumbers. Thus, for example, a display may be chosen to render 2 yellowstates (since the human visual system is relatively insensitive tospatial variation in blue light, the absence of which corresponds to theyellow subtractive primary color) and 2⁴=16 states of each of magentaand cyan. The waveform driving the display would be required to render34 different states in total, but would be able to address 2⁹=512different colors.

In one aspect, this invention provides a color display in which a singlepair of electrodes are used to address independently more than one layerof electrophoretic or similar electro-optic material. Such a colordisplay can provide independent, or at least partially independentaddressing of more than one primary color using a single pair or singleset of electrodes (for example, the single set of electrodes can be anactive matrix backplane and a single continuous counter-electrode. Atleast one of the layers of electrophoretic or similar material mayoperate in shutter mode (as defined above).

Shutter mode electrophoretic displays can be used as light modulators.Light modulators represent a potentially important market forelectro-optic media. As the energy performance of buildings and vehiclesbecomes increasingly important, electro-optic media can be used ascoatings on windows (including skylights and sunroofs) to enable theproportion of incident radiation transmitted through the windows to beelectronically controlled by varying the optical state of theelectro-optic media. Effective implementation of such“variable-transmissivity” (“VT”) technology in buildings is expected toprovide (1) reduction of unwanted heating effects during hot weather,thus reducing the amount of energy needed for cooling, the size of airconditioning plants, and peak electricity demand; (2) increased use ofnatural daylight, thus reducing energy used for lighting and peakelectricity demand; and (3) increased occupant comfort by increasingboth thermal and visual comfort. Even greater benefits would be expectedto accrue in an automobile, where the ratio of glazed surface toenclosed volume is significantly larger than in a typical building.Specifically, effective implementation of VT technology in automobilesis expected to provide not only the aforementioned benefits but also (1)increased motoring safety, (2) reduced glare, (3) enhanced mirrorperformance (by using an electro-optic coating on the mirror), and (4)increased ability to use heads-up displays. Other potential applicationsof VT technology include privacy glass and glare-guards in electronicdevices.

VT media capable of being used as VT windows have been demonstrated andare described in the patent literature; see, for example, U.S. Pat. No.7,327,511; and U.S. Patent Applications Publication Nos. 2006/0038772;2007/0146310; and 2008/0130092. However, there are certain remainingproblems in such VT media. Firstly, it is difficult to achieve in thesame medium acceptable levels of image stability (i.e., stabletransmission) and haze. In practice, VT windows need high imagestability of the order of several hours, since users do not wish to havetheir windows adjusting their transmission levels every few minutes. Asdiscussed above (see the aforementioned U.S. Pat. No. 7,170,670), theimage stability of electrophoretic media can be increased by dissolvingor dispersing a high molecular weight polymer in the fluid in the fluidin which the electrophoretic particles (typically carbon blackparticles) are dispersed; the aforementioned 2007/0146310 recommends apolystyrene dispersed in a mixture of limonene and a partiallyhydrogenated terphenyl as a fluid for used in VT electrophoretic media.The effect of adding the polymer to the fluid is to increase theflocculation of the electrophoretic particles once the particles areaggregated together, and thus to increase the image stability.Unfortunately, the polymer also serves to increase particle flocculationeven when the electrophoretic particles are intended to be dispersedthroughout the fluid, and the resultant increase in particle aggregationand thus effective particle size, substantially increases the opticalhaze of the medium; the particle size of the aggregated particles islarge enough to cause substantial scattering of visible light, eventhough the individual particles themselves are sufficiently small thatthey will cause little scattering. The light scattering responsible forhaze depends upon both the particle size and the difference inrefractive index between the electrophoretic particles and thesurrounding fluid. To date, no black pigment has been identified whichhas a refractive index close enough to that of the fluids typically usedin electrophoretic media (or close enough to that of the polymeric phasewhich typically surrounds the fluid, as discussed above) to reduce hazeto an acceptable level, and no mechanism for increasing image stabilityto the degree considered desirable for commercial sale has beenidentified which does not increase haze to an undesirable level.

Another problem with prior art VT electrophoretic media (and similarelectro-optic media such as electrochromic media) is their inability tovary hue; in other words, the colors capable of being displayed by suchmedia fall on a line between their endpoint colors (a transparent statebeing regarded as a “color” for purposes of the present application),and the media do not have a color gamut volume. For example, the colorsobtainable from the VT media described in the aforementioned U.S. Pat.No. 7,327,511; and U.S. Patent Applications Publication Nos.2006/0038772; 2007/0146310; and 2008/0130092 vary from black to clear,while electrochromic media typically vary from blue-purple to clear.(Provision of color in VT media may be useful either in enabling thelight within a room equipped with VT windows to be varied, or inenabling the use of VT as one layer in a multi-layer display, asdiscussed in detail below.) Neither type of media can produce additionalcolors without the addition of a color filter array, and the use of amulti-pixel drive method, typically using a passive or active matrixbackplane. Such a backplane inevitably reduces optical transmissionthrough the VT medium and is far more expensive than the simpleelectrode used in a single pixel VT display

Accordingly, there is still a need for a VT medium which can provide thehigh image stability desirable in commercial VT displays in combinationwith low haze. There is also still a need for VT media which can providea substantial color gamut. In one aspect, the present invention seeks toprovide solutions to both these problems.

SUMMARY OF INVENTION

In one aspect, this invention provides an electro-optic displaycomprising at least first and second layers of electro-optic material,the first layer being capable of displaying at least one optical statewhich cannot be displayed by the second layer, the display furthercomprising a first electrode disposed on one side of the first andsecond layers, and a second electrode disposed on the opposed side ofthe first and second layers from the first electrode, there being noelectrode between the first and second layers. (The term “electrode” isused herein in its conventional meaning in the electro-optic display artto mean a conductive material the electrical potential of which can becontrolled by being connected to a source of known potential, includingground. Thus, for purposes of this application, a conductive materialnot arranged to be connected to any source of known potential is not anelectrode.)

Such a display may further comprises a third layer of electro-opticmaterial, the third layer being capable of displaying at least oneoptical state which cannot be displayed by the first and second layers,the second electrode being disposed on the opposed side of the first,second and third layers from the first electrode, there being noelectrode between the second and third layers. The three layers ofelectro-optic material may comprise a set of subtractive primary colors,for example cyan, magenta and yellow pigments; it will readily beapparent that these colors may be distributed among the first, secondand third layers in any order. (Alternatively, the displays of thepresent invention may use more than three primary colors, or use primarycolors that are not the conventional subtractive primary colors). Thelayer containing the yellow pigment may be arranged to have a smallernumber of gray levels than the layers containing the cyan and magentapigments. In one form of the invention, the electro-optic display has aviewing surface through which an observer views the display, and the twoelectro-optic layers closest to the viewing surface contain cyan andyellow pigments, in either order.

At least one layer of electro-optic material may comprise anelectrophoretic material comprising a plurality of charged particlesdispersed in a fluid and capable of moving through the fluid onapplication of an electrical field to the layer. Such an electrophoreticlayer may be unencapsulated or may comprise a microcavityelectrophoretic material. The charged particles in at least one of thelayers (and preferably both the first and second layers) may be movablebetween a first optical state, in which the pigment particles occupysubstantially the whole area of each pixel, and a second optical state,in which the pigment particles occupy only a minor proportion of thearea of each pixel.

When the first and second electro-optic layers (and the thirdelectro-optic layer if present) are both electrophoretic layers,independent control of the pigments in these layers may be facilitatedby controlling a variety of physical parameters of the two layers. Forexample, the yield stresses of the fluids in the first and second layersmay differ; the sizes of the microcavities in the first and secondlayers may differ; the particles in the first and second layers maydiffer in at least one of size, shape and electrical conductivity; orthe fluids in the first and second layers may differ in at least one ofviscosity and electrical conductivity.

A preferred form of electro-optic display of the present invention has aviewing surface at or adjacent the surface of the first electro-opticlayer remote from the second electro-optic layer, and has a thirdelectro-optic layer on the opposed side of the second electro-opticlayer from the viewing surface, the third electro-optic layer comprisingfirst and second types of particles of differing colors disposed in afluid and capable of moving through the fluid on application of anelectric field to the third electro-optic layer, the first and secondtypes of particles being of differing electrophoretic mobility. Thefirst type of particles in the third electro-optic layer may be white.The particles in the first and second electro-optic layers may be yellowand cyan, in either order, and the second type of particles in the thirdelectro-optic layer may be magenta.

At least one of the first and second electrodes in the electro-opticdisplay of the present invention may occupy only a minor proportion ofthe area of a pixel. In a microcavity display of the present invention,the walls of the microcavity may have a higher electrically conductivitythan the phase comprising the plurality of charged pigment particlesdispersed in the fluid. One of the electro-optic layers may have atleast two stable states, while a second electro-optic layer may haveonly one stable state.

In another aspect, this invention provides an electro-optic displaycomprising first and second layers of cavities disposed adjacent oneanother with no electrode between the first and second layers, thesections of each of first and second layers of cavities lying adjacentthe other layer being of substantially pyramidal form tapering towardsthe other layer, at least one of the first and second layers comprisingcolored particles dispersed in a fluid and capable of moving through thefluid on application of an electric field to the display. In such adisplay, the first and second layers may be formed of deformablecapsules. One of the first and second layers may be free from coloredparticles.

In another aspect, this invention provides an electro-optic displaycomprising at least first, second and third layers of electro-opticmaterial, the display further comprising a first electrode disposed onone side of the first, second and third layers, and a second electrodedisposed on the opposed side of the first, second and third layers fromthe first electrode, there being no electrode between the first andsecond layers, or between the second and third layers, the first, secondand third layers having the following properties:

-   -   (a) the first layer 1 has a voltage and/or impulse threshold, is        state stable, and its color depends upon the polarity of the        applied voltage;    -   (b) the second layer has either a lower threshold than the first        layer, is state stable, and its color depends upon the polarity        of the applied voltage; and    -   (c) the third layer has no threshold, is not state stable,        switches faster than the second layer, and reaches the same        state whether driven with a positive or a negative impulse and        relaxes to its opposite extreme when no potential is applied.

Such a display may be driven by a method comprising:

(a) applying a high voltage to set the first layer to the desired color;

(b) applying a lower voltage than in step (a) to set the second layer tothe desired color; and

(c) allowing the third layer to relax to its desired color.

In another aspect, this invention provides a microcavity electrophoreticdisplay comprising walls defining at least one cavity, the cavitycontaining a fluid and first, second and third types of particlesdispersed in the fluid, each of the first, second and third types ofparticles having an unshuttered state, in which the particles occupysubstantially the entire area of the microcavity, and a shuttered state,in which the particles occupy only a minor proportion of the areas ofthe microcavity, the first, second and third particles being ofdiffering colors and differing in dielectrophoretic and/orelectro-osmotic properties such that the first, second and third typesof particles can be moved between their unshuttered and shuttered statesindependently of one another. In such a display, the colors of thefirst, second and third types of particles are such that when all threetypes of particles are in their unshuttered states the display appearssubstantially black.

This aspect of the present invention is based upon the discovery of arange of colored organic pigments (i.e., having a color other thanblack) which provide image state stability without the need of apolymeric additive in the fluid of the electrophoretic medium. While theexact image state stability mechanism is not completely understood, itappears that these pigments themselves form loose aggregates whensuspended in a fluid. This loose aggregate exhibits a yield stress: whenthe pigment dispersion is sufficiently concentrated, a gel is formed.With slight agitation, the gel is broken, resulting in a low viscosityfluid. Some of these pigments have refractive indices close to that ofthe preferred internal phase fluid and polymer phase surrounding thefluid, resulting in a low haze dispersion, even in the flocculatedstate. By appropriate blending of various colored pigments, a mediumhaving a black optical state can be created.

Many such blends of organic pigments can also be made to display asubstantial color gamut. By careful selection of the organic pigmentsused, the differently colored organic pigments present can be chosen tohave substantial different dielectrophoretic mobilities, so that bymanipulating the frequency and voltage applied to the electrophoreticmedium, each pigment individually can be made to be in a dispersed stateor in a packed state, i.e., the different organic pigments can be madeto “shutter” independently of one another. Having a particular organicpigment in its dispersed state allows that pigment to absorb thecorresponding color of transmitted light; the more uniform thedispersion, the greater the absorption. On the other hand, having aparticular organic pigment in its packed state minimizes the fraction ofthe area of the electrophoretic medium where that pigment is located,thus minimizing the absorption by that pigment. It will readily beapparent by controlling the “shuttering” of differently coloredparticles independently of one another, a substantial color gamut can bedisplayed.

Such a multi-colored VT medium can be used directly as a full colordisplay; if a reflective display is desired, a reflector can of coursebe placed behind the medium (i.e., on the opposed side of the mediumfrom that viewed by the user). Alternatively, such a multi-colored VTmedium can be used as a substitute for a conventional “static” colorfilter array, and used in combination with a monochrome reflectivemedium (either an inherently reflective medium or a transmissive mediumprovided with a reflector); this monochrome medium need not be anelectrophoretic medium but could be any known type of electro-opticdisplay. Such a dual layer display would have the advantage of allowingthe multi-colored VT medium to control only the color of the variouspixels or sub-pixels of the display, while the monochrome medium wouldcontrol the brightly of each pixel or sub-pixel. Requiring themulti-colored VT medium to control only color lessens the demands uponindependent control of the multiple pigments used in the VT medium,since no longer is the VT medium required to provide all colors at alllevels of saturation.

The VT media of the present invention may be of any of the types ofelectrophoretic media discussed above. Thus, the VT media may beunencapsulated, encapsulated, microcell or polymer-dispersed media.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 of the accompanying drawings, as already mentioned, illustrates asimple color cube.

FIG. 2A is an idealized side elevation of a single layer of driedmicrocapsules formed on a flat surface.

FIG. 2B is a top plan view of the layer of microcapsules shown in FIG.2A.

FIG. 2C is an idealized side elevation, similar to that of FIG. 2A, of adouble layer of microcapsules formed on a flat surface.

FIG. 2D is a top plan view of the double layer of microcapsules shown inFIG. 2C.

FIG. 3A is a schematic side elevation of two capsules in which pigmentshuttering is effected using concentrator electrodes.

FIG. 3B is a schematic side elevation, similar to that of FIG. 3A, oftwo capsules which use side wall shuttering of pigment.

FIG. 3C is a schematic side elevation, similar to those of FIGS. 3A and3B, showing how geometric pigment shuttering may be effected in a doublelayer of capsules similar to that shown in FIGS. 2C and 2D.

FIGS. 3D and 3E show respectively the non-shuttered and shuttered statesof the second layer of capsules in a double layer of capsules similar tothat shown in FIGS. 2C and 2D.

FIG. 4 is a schematic cross-section through a single capsule used in adisplay of the present invention and illustrates pigment dispersioneffected by capsule wall charging.

FIGS. 5A, 5B and 5C illustrate, in idealized form, three different waysin which shuttered pigment particles may be packed in a microcavity, andthe corresponding transmission efficiency for a double pass of lightthrough the microcavity.

FIG. 6 is a graph showing the transmission efficiencies of the threepigment arrangements of FIGS. 5A, 5B and 5C as a function of the volumefraction of pigment in the cavity.

FIGS. 7A, 7B and 7C, which are reproduced from Todd Squires and MartinBazant, J. Fluid Mech., (2004) 509, 217-252, show in cross-section aspherical pigment particle surrounded by a solvent that contains acharge control agent, and illustrates the forces acting on the pigmentparticle in various types of applied electric fields.

FIGS. 8A-8D illustrate various states of a two electro-optic layerdisplay of the present invention, and show the manner in which twolayers can be made to shutter independently using a common waveform.

FIGS. 9A and 9B are projections on the a*b* plane of the La*b* colorspace and illustrate the color changes occurring during the operation ofone display of the present invention in which two layers switch at thesame rate.

FIGS. 10A and 10B are projections similar to those of FIGS. 9A and 9Brespectively but illustrate the color changes occurring in a display inwhich the two layers switch at different rates.

FIG. 11 is a schematic side elevation, similar to those of FIGS. 8A-8Dbut showing a three-layer display of the present invention in which onelayer contains no pigment particles.

FIGS. 12A and 12B are schematic side elevations, similar to that of FIG.11, showing two further three-layer displays of the present invention.

FIGS. 12C and 12D show two different states of a further three-layerdisplay of the present invention.

FIGS. 13A and 13B illustrate the color performance of the display testedin Example 10 below.

FIGS. 14A and 14B illustrate the color performance of the display testedin Example 11 below.

FIGS. 15A and 15B illustrate the color performance of the display testedin Example 12 below.

FIG. 16 (which is similar to FIGS. 13B, 14B and 15B) illustrates thecolor performance of the display tested in Example 13 below.

FIG. 17 illustrates the colors obtained in Example 3 below.

DETAILED DESCRIPTION

As already mentioned, in one aspect this invention provides anelectro-optic display comprising at least first and second layers ofelectrophoretic material, each of which comprises a plurality of chargedparticles dispersed in a fluid and capable of moving through the fluidon application of an electrical field to the layer. The first layer ofelectrophoretic material is capable of displaying at least one opticalstate which cannot be displayed by the second layer. The display furthercomprises a first electrode disposed on one side of the first and secondlayers, and a second electrode disposed on the opposed side of the firstand second layers from the first electrode; no electrode is presentbetween the first and second layers. Typically, the electrophoreticdisplay will further comprise a third layer of electrophoretic materialcomprising a plurality of charged particles dispersed in a fluid andcapable of moving through the fluid on application of an electricalfield to the layer. The third layer is capable of displaying at leastone optical state which cannot be displayed by the first and secondlayers. The second electrode is disposed on the opposed side of thefirst, second and third layers from the first electrode, and noelectrode between the second and third layers, the single pair (or set)of electrodes being used to control all three layers at least partiallyindependently of one another.

The basic concept behind the present invention is perhaps most easilyappreciated by considering a three-layer display such as that shown inFIG. 12A of the accompanying drawings. The electro-optic layer closestto the viewing surface (the surface through which an observer views thedisplay) has two different optical states, a “dispersed” or“unshuttered” state in which color is present over the whole area of apixel, and a “concentrated” or “shuttered” optical state in which thecolor is absent from the major proportion of the area of the pixel andis present (if at all) only in a minor proportion of the area of thepixel. The second electro-optic layer (which lies behind the firstelectro-optic layer, as seen by the observer) operates in a mannersimilar to the first layer but uses a different color.

The third electro-optic layer (the one remote from the observer) of sucha three-layer display may operate in the same manner as the first andsecond electro-optic layers using a third color; the first, second andthird colors are normally chosen to form a set of subtractive primaries.If the third layer does act in this manner, a reflector will bepositioned behind the display to reflect light which has passed throughthe three electro-optic layers back through those layers to theobserver. More commonly, however, the third electro-optic layer is achosen so that it can display either one of two colors, namely the thirdsubtractive primary or white; the third electro-optic layer may be, forexample, a conventional dual particle electrophoretic layer such asdescribed in many of the aforementioned E Ink patents and applications.

The three electro-optic layers of the display are disposed between asingle set of electrodes and an appropriate display controller is usedto enable the three electro-optic layers to be driven at least partiallyindependently of one another. In a preferred form of the display of thepresent invention in which the first and second electro-optic layers areshutter mode electrophoretic layers and the third electro-optic layer isa dual particle electrophoretic layer, the third layer can be drivenusing a direct current driving method, while the first and second layersare driven by different shuttering drive methods, as described in detailbelow.

Hereinafter for convenience, a material that selectively absorbs lightis referred to as a “pigment”, which term should be interpreted toinclude dyes, photonic crystals, etc., capable of selectively absorbinglight. In embodiments of the invention intended to provide full colorimaging using three subtractive primary pigments, light will typicallybe selectively filtered through at least two pigments before beingreflected back to the viewer. The third pigment may be transparent orreflective, as described in more detail below. It is therefore necessaryfor at least two of the pigments used in the present invention to belight-transmissive and not substantially back-scattering. Thus, forexample, a magenta pigment is intended to absorb green light but mustpass blue and red light to underlying layers. In regions where greenlight is not intended to be absorbed, it is necessary that the pigmentnot be present in the optical path. One way in which such removal of thepigment from the optical path may be achieved is to concentrate thepigment in only a (minor) portion of the pixel area, thus reducing itscovering power. When magenta color is desired, the pigment is spreadover the whole pixel area to enable the maximum amount of light to beabsorbed. The process of spatially concentrating the pigment to reduceits areal covering power is referred to as “shuttering” the pigment.

Numerous methods can be used for shuttering pigments in response to anapplied electric field, as described in detail below. As alreadymentioned, the displays of the present invention may make use ofelectro-optic layer formed from microcapsules, which may be coated inroll-to-roll processes. Alternatively, the electro-optic layers may usemicrocells, microcups or wells such as are known in the art. Althoughthe invention will hereinafter primarily be described with regard toelectro-optic layers using microcapsules, it is believed that thoseskilled in the technology of electro-optic displays will have nodifficulty adapting the microcapsule based structures described to othermethods for spatially segregating the pigment containing phases.

As already indicated, the present invention is directed to electro-opticdisplays in which multiple pigments are controlled by a single set ofelectrodes. Since the electric fields present between a single set ofelectrodes are substantially the same regardless of whether one or aplurality of electro-optic layers are present between those electrodes,it will be appreciated that the reaction of various pigments to theelectric fields generated the single set of electrodes will in mostcases be substantially the same regardless of whether the pigments arepresent between the electrodes in the same or different electro-opticlayers. Accordingly, various embodiments of the present invention can beproduced depending upon whether various pigments are present in the sameor different electro-optic layers. Although the present invention willprimarily be described with reference to embodiments of the invention inwhich each electro-optic layer (except one) contains only a singlepigment, depending upon the exact driving methods used, all the pigmentsused may be contained in a single layer, or two pigments may becontained in one layer and a third in a different layer, and the meansused to shutter the pigments may differ in different layers. Adescription is given below of one display of the present invention inwhich three pigments are present in a single layer of capsules.

Certain “geometric” methods for shuttering pigments rely, in some cases,upon the self-assembly of coated layers of microcapsules. Hence, apreliminary discussion of such coated layers is desirable. As discussedin many of the aforementioned E Ink patents and applications, andespecially in U.S. Pat. Nos. 6,067,185; 6,392,785; 7,109,968; and7,391,555, in practice prepared by forming an emulsion in which thediscontinuous phase comprises droplets of an electrophoretic internalphase that comprises at least one pigment and a fluid (which istypically a low polarity, substantially water-immiscible hydrocarbon),normally with the addition of a charge control agent. The continuousphase of the emulsion comprises an aqueous solution of a polymer,typically gelatin. Polymeric material is deposited onto the surface ofthe droplets by, for example, formation of a coacervate of the gelatinand a second polymer, typically acacia, to form a thin capsule wall thatmay optionally be cross-linked, for example with an aldehyde. Theresultant deformable microcapsules are spheres of approximately 20-100μm in diameter. When such microcapsules are coated on a flat surface ata controlled coverage, they form essentially a monolayer of capsules.When this monolayer is dried, the capsules tend to contract vertically(i.e., perpendicular to the surface on which they are coated) and expandlaterally to form oblate spheroids. Eventually, as the capsules expandlaterally, their sidewalls come into contact with each other, and thecapsules deform into polyhedral prisms whose shapes are similar to thoseformed by cells in foams. Ideally, a single layer of capsules will forma “honeycomb” (a two dimensional hexagonal lattice) of hexagonal prismswhose side walls, viewed in projection, meet at 120 degree angles, asshown (in idealized form) in FIG. 2A. (In practice, the microcapsulesvary somewhat in size, and photomicrographs of dried single microcapsulelayers typically show honeycombs similar to that illustrated in FIG. 2B,but with each microcapsule having from 4 to 8 neighbors. For reasonswhich will appear below, shuttering of pigments in the displays of thepresent invention is not greatly affected by such deviations from anideal honeycomb of FIG. 2B). Also, as shown in FIG. 2A, typically thefaces of the capsules, that are in contact with the planar substrateonto which the capsules are coated, will conform to the flat surface,while the exposed face of each capsule will adopt a curved, “domed”shape.

When a second layer of microcapsules is coated on top of the firstlayer, surface tension forces leading to minimization of surface energytend to cause deformation of the domed upper surfaces of the capsules inthe first layer into a foam-like geometry as illustrated schematicallyin FIG. 2C. In this geometry, the upper portion of each capsule in thefirst layer has a substantially pyramidal shape in which the pyramidalportion are substantially flat and the intervening edges aresubstantially straight lines, four of which meet at each vertex at thetetrahedral angle of 109.5 degrees. For a detailed description of thegeometry of foams, see for example “Foams: Theory, Measurements, andApplications”, R. K. Prud'homme and S. A. Khan, eds., Marcel Dekker,Inc., 1996. Classically, the shape of each cell in a monodispersed foamis a semi-regular solid (essentially a truncated octahedron) with 14faces. Note that the lower portions of each capsule in the second layeralso has a substantially pyramidal shape, so that the pyramidal portionsof the first and second layers fit together with the lowest vertex ofeach pyramidal section in the second layer filling into the recessbetween three pyramidal sections in the second layer. As explainedbelow, the pyramidal sections of such double layers of capsules areimportant in one type of pigment shuttering. It will be appreciated thatif a third layer of capsules is coated over the second layer, theinterface between the second and third layers will display the same typeof interpenetrating pyramidal sections as the interface between thefirst and second layers.

FIGS. 3A-3E illustrate various forms of pigment shuttering which may beemployed in the displays of the present invention. FIG. 3A illustratesthe shuttered optical state of a display which uses concentratorelectrodes 102. Such concentrator electrodes are small electrodes whichoccupy only a small fraction of the area of each pixel so that when anappropriate voltage is applied to the concentrator electrodes, thepigment is attracted to the concentrator electrodes and thus onlyoccupies a small fraction of the area of each pixel, i.e., the pigmentis shuttered.

Concentrator electrodes may be patterned electrodes by which the displayis addressed, for example grids of conductive material such as silver orgold that may be patterned onto a substrate by printing or lithographicmethods, or continuous conductors that are masked with patterns ofdielectric material. Concentrator electrodes may also beindividually-addressable electrodes that are associated with an array ofthin-film transistors. Alternatively, discrete, isolated conductiveparticles that are not directly electrically-addressed may beincorporated into a layer or layers within the display.

FIG. 3B illustrates geometric/capsule wall pigment shuttering in thefirst layer of a double capsule layer of the type illustrated in FIGS.2C and 2D. Because of the pyramidal form of the upper section (asillustrated) of the first layer of capsules, when the potentials appliedto electrodes 104 and 106 are arranged so that the pigment particles areattracted towards electrode 104, the pyramidal form of the uppersections of the capsules in the first layer will cause the pigment toform pigment packs 108 which occupy only the area around the uppermostvertex of each pyramidal section, and thus occupy only a small fractionof the area of each pixel, i.e., the pigment is shuttered.

Geometrical/capsule wall shuttering can be achieved naturally by the useof more than one layer of capsules whose wall materials are moreelectrically conductive than their internal phase. For example, agelatin capsule wall typically has a conductivity on the order of 10⁻⁷S/m (although this value is strongly dependent upon the degree ofhydration of the polymeric material comprising the capsule wall). Anelectrophoretic internal phase typically has a conductivity much lessthat this, on the order of 10⁻⁸ S/m. Thus, the capsule wall of one layerof capsules can act as a concentrator “electrode” for a second layer ofcapsules. In addition, a geometrical shutter may be provided by thepyramidal shape that may be adopted by the capsule, as described above.If needed, geometrical shutters may be achieved by templated coating ofcapsules into, for example, V-shaped grooves as described in U.S. Pat.Nos. 6,130,774 and 6,172,798. Geometrical shutters may also befabricated using photolithographic or embossing methods, or othermethods that are well-known in the art.

FIGS. 3D and 3E illustrate the use of geometric shuttering in the secondlayer of a double capsule layer of the type illustrated in FIGS. 2C and2D. As shown in FIG. 3D, when the potentials applied to electrodes 104and 106 are arranged so that the pigment particles are attracted towardselectrode 104, because of the domed shaped of the upper portions of thesecond layer of capsules, the pigment spreads out over the whole area ofthe domed portion, and thus the entire pixel area, and is not shuttered.On the other hand, when the potentials applied to electrodes 104 and 106are arranged so that the pigment particles are attracted towardselectrode 106, the pyramidal form of the lower sections of the secondlayer of capsules causes the pigment to form small pigment packs 110adjacent the lowermost vertex of each pyramidal section, and thus occupyonly a small fraction of the area of each pixel, i.e., the pigment isshuttered.

FIG. 3C illustrates side wall shuttering. In this form of shuttering, inthe shuttered state the pigment is moved laterally parallel to the planeof the electrodes so that it forms pigment 112 adjacent the side wallsof the capsules.

Other methods for shuttering, such as use of anisotropic particles, forexample, needles or plates that may be oriented with their principalaxes either perpendicular or parallel to the plane of the display, orcolor-changing pigments, or swelling and deswelling of gels, or othersimilar methods that are known in the art, may also be used in thedisplays of the present invention.

As will readily been seen from the discussion of FIGS. 3A, 3B, 3D and 3Eabove, pigment shuttering using concentrator electrodes orgeometrical/capsule wall shuttering may be achieved with direct currentaddressing of a display, in which the pigment motion is in the directionof the applied electric field. Side-wall shuttering may be achieved byaddressing the display with alternating, in which case net pigmentmotion may be in a direction perpendicular to the applied electricfield, as described in more detail below, such that pigment in theinterior of the capsule is deposited in an equatorial “belt” around thecapsule.

The displays of the present invention obviously require some method fordispersing the pigment from its shuttered state to its unshutteredstate, in which the pigment occupies substantially the entire area of apixel. One method for such pigment dispersion that is especiallypreferred for use in conjunction with concentrator electrode orgeometrical/capsule wall shuttering is illustrated schematically in FIG.4, which illustrates the use of a combination of DC-addressing and acharged capsule wall being used to move a pigment perpendicular to anapplied electric field. In the capsule shown in FIG. 4, the loading ofpigment is very low (around 1% of the volume of the capsule) such thatit may be concentrated into a very small region (shown as the lowermostvertex of the lower pyramidal section of the capsule in FIG. 4). Uponapplication of an electric field, the pigment would normally migratefrom this vertex to the opposed vertex (i.e., vertically upwardly asillustrated in FIG. 4), where it would again be concentrated in a verysmall region adjacent the uppermost vertex of the upper pyramidalsection of the capsule. By providing an attractive force between thepigment particles and the capsule wall, a perpendicular component (i.e.,a component horizontally as illustrated in Figure) may be added to theelectrical force applied to the particles, and so that the particlesspread out laterally from the small region they occupy in theirshuttered position. The necessary attractive force may be electrostatic.Thus, in some embodiments of the present invention, it is preferred thatthe pigment particles and the capsule wall bear charges of oppositepolarity. (Obviously, this aspect o the invention cannot generally beapplied to capsules which contain particles bearing charge of bothpolarities, and is best suited to capsules containing only one type ofparticle or “same polarity dual particle capsules, as described forexample in U.S. Pat. No. 6,870,661) For example, if gelatin/acacia isused to form the capsule wall, and a charging agent such as Solsperse17000 (available from Lubrizol) is used, the capsule wall may attain anegative charge and a pigment that attains a positive charge with thesame charging agent is preferred. Other methods that may provideattraction between the pigment particles and the capsule wall includethe use of flocculating agents, especially depletion flocculants. Theresult of using such capsule-wall attraction is that the pigment will beessentially invisible at either extreme of a DC pulse, but visibleduring the transition from one concentrated state (at the top of thecapsule) to another concentrated state (at the bottom of the capsule) orvice versa. The pigment may be trapped in what would be a transientstate in simple DC driving by driving with AC having a frequency ofbetween about 30 and 50 Hz, and applying a DC offset to the AC drive.

The present invention is not confined to use of a mobile charged pigmentwith a capsule wall bearing a charge of the opposite polarity butextends to the use of such a pigment with any fixed surface bearing acharge of the opposite polarity. The fixed surface acts to constrain themotion of the pigment in an applied field. The medium containing thepigment and the surface need not be encapsulated.

The shuttering mechanisms described above may be combined withconventional switching of pigments using DC addressing to give rise toparticle motion within a capsule parallel to the applied field asalready mentioned with regard to FIGS. 3D and 3E. In this case is may bedesirable that geometrical/capsule wall shuttering not occur, and thiscan be ensured by incorporating more pigment into the capsule than canbe concentrated in a groove or crevice. Such conventionalelectrophoretic switching may involve a single pigment moving through adyed liquid, dual pigments of the same or opposite charge, orcombinations of multiple pigments and dyed fluids, as described in theaforementioned E Ink patents and applications.

Shuttering methods of all types tend to impose limitations on the volumefraction of pigments particles which can be present in anelectrophoretic medium. It is desirable that at least about 85 percentof light be transmitted through a layer of shuttered pigment in a singlepass, i.e., the pigment should not absorb more than about 15 percent ofthe light; this corresponds to a reflection efficiency of about 70percent for a double pass through the shuttered layer such as willnormally occur in the displays of the present invention. FIGS. 5A, 5Band 5C show, in simplified form, three different possible forms ofpacking of pigment particles using some idealized shuttering methods. Inthe following simplified analysis, light losses due to (inter alia)total internal reflection, scattering except by a Lambertian reflectorbehind the cavity, and interfacial reflections are ignored. FIG. 5Apigment is deposited onto the side-walls of a cavity whose walls areperpendicular to the plane of the display. Assuming that the pigmentabsorbs all incident light and that light passes once through thecavity, is reflected in a Lambertian fashion, and then passes a secondtime through the cavity, the proportion of light absorbed is (1−α)²where α is the volume fraction of the pigment within the cavity. In FIG.5B, the pigment is collected in a half-cylinder on one face of thecavity parallel to the viewing surface of the display (which is assumedto be horizontal in FIGS. 5A-5C). The form of pigment collection willtypically be appropriate for collection at a concentrator electrode inthe absence of any geometrical shuttering. FIG. 5C shows concentrationof pigment into a spherical shape.

FIG. 6 is a graph showing the estimated double pass transmissionefficiency for each shuttering geometry shown in FIGS. 5A, 5B and 5C asa function of the volume fraction of the pigment within the cavity.Assuming that a double pass efficiency of 70 percent is acceptable, FIG.6 shows that this requires not more than about 1 percent by volume ofpigment for a hemicylindrical pigment geometry (FIG. 5B), and not morethan about 5 percent by volume for a spherical pigment geometry (FIG.5C). In view of these low acceptable pigments concentrations(“loadings”) it is preferred that pigments used for shuttering have themaximum possible extinction coefficient so that they can be used in theleast possible loading.

Such volume fraction limitations for shuttered pigments also imposesconstraints on the preferred particle size of such pigments. Particlesthat are randomly arranged on a surface cover that surface lessefficiently than an ordered, close-packed monolayer, such thatapproximately two monolayer-equivalents of particles are required for anarea coverage of 90 percent. In addition, for a typical dye having anextinction coefficient of 50,000 L/mole/cm, a specific gravity of 1.5and a molecular weight of 500, a perfectly-packed layer of 70 nmthickness is required for an optical density of 1 (i.e., 90 percentabsorption of light). It is therefore preferred that for such a dye, thediameter of the pigment particle be on the order of half this thicknessor less. In practice, pigment particle sizes of less than about 100 nmdiameter are preferred for concentrator electrode andgeometrical/capsule wall shuttering. Larger particles may be hiddenusing side-wall shuttering since this method permits a larger loading ofpigment for the same degree of shuttering, as described above.

Additional optical losses may occur if the positions at which thepigments are shuttered in separate layers do not overlie one another(i.e., are not registered, as would be the case when the pigments arelocated in different layers in a display having multiple layers ofcapsules). If the absorption spectra of the pigments overlap, some lightof a wavelength absorbed by both pigments may be absorbed in one area ofa pixel by a first shuttered pigment in a first layer and in anotherarea of the same pixel by a second shuttered pigment in a second layer.This problem can be avoided by removing spectral overlap in theshuttering pigments so that there is no wavelength absorbed by twopigments. Thus, in a display when two pigments are shuttered and a thirdis switched by conventional electrophoresis, it is preferred that thetwo shuttered pigments be yellow and cyan (whose absorption spectra areboth designed to pass green light, and which therefore do notsignificantly overlap).

As mentioned above, AC addressing may be used to shutter pigments. Arich variety of phenomena occur when AC addressing is employed,including induced-charge electro-osmosis and electrophoresis andinduced-dipole effects such as dielectrophoresis and particle chaining.Which behavior occurs is affected by the field strength and frequencyapplied and by properties of the components in the capsule internalphase, as will now be described in more detail.

FIG. 7A shows a schematic cross-section through a spherical pigmentparticle surrounded by a solvent that contains a charge control agent(CCA). The CCA is typically a surfactant-like molecule comprising ionicgroupings, hereinafter referred to as “head groups”. At least one of thepositive or negative ionic head groups is preferably attached to anon-polar chain (typically a hydrocarbon chain) that is hereinafterreferred to as a “tail group”. It is thought that the CCA forms reversemicelles in the internal phase and that a small population of chargedreverse micelles leads to electrical conductivity in the internal phase.The CCA also is thought to adsorb onto the surfaces of the pigmentparticles and onto the interior walls of the capsules. Collectively theCCA and the reverse micelles mediate charging of all surfaces (on theparticle and the capsule wall) in the internal phase of theelectrophoretic medium.

It is thought that a particle with immobilized charge on its surfacesets up an electrical double layer of opposite charge in the surroundingfluid. Ionic head groups of the CCA may be ion-paired with chargedgroups on the particle surface, forming a Stern layer of immobilizedcharged species. Outside this layer is a diffuse layer comprisingcharged micellar aggregates of CCA. In conventional DC electrophoresisan applied electric field exerts a force on the fixed surface chargesand an opposite force on the mobile counter-charges, such that slippageoccurs within the diffuse layer and the particle moves relative to thefluid. The electric potential at the slip plane is known as the zetapotential.

Induced-charge electro-osmosis (hereinafter abbreviated “ICEO”, butknown as “AC electro-osmosis”) is a similar phenomenon but occurs inresponse to induced charge rather than fixed surface charges. It isdescribed in V. A. Murtsovkin, Colloid J., 58, 341-349 (1996) and in aseries of papers by H. Morgan and co-workers (see, for example, J.Colloid Interface Sci., 217, 420-422 (1999) and Phys. Rev. E, 61,4011-4018 (2000)), and has more recently been reviewed in detail bySquires and Bazant (J. Fluid Mech., 509, 217-252 (2004). In ICEO, anexternally applied electrical field induces a polarization in thevicinity of a surface and simultaneously drives the resultingelectro-osmotic flow. This creates a flow velocity that is nonlinear inthe applied field strength. In the presence of an applied electric fieldan induced dipole may be set up (see FIG. 7A), the magnitude of whichdepends upon particle bulk and surface properties includingconductivity, dielectric constant, size and shape. This induced dipolein turn causes a flow of ionic species (probably micelles) in the fluidthat sets up a corresponding double layer of opposite charge (FIG. 7B).An electro-osmotic flow is then driven (FIG. 7C) such that the fluid isdrawn in from the poles and expelled at the equator. The direction ofthe fluid flow is the same irrespective of the polarity of the appliedelectric field, and the flow can thus be driven by an applied ACpotential. In the case of a spherical particle in a uniform field, noparticle motion would result (since the flows are symmetrical). Inpractice, however, pigment particles are not perfectly sphericallysymmetrical and jets of the mobile phase may be set up that inducechaotic motion of the particles.

Bazant has estimated the charging time for establishment of the doublelayer (FIG. 7B) as:

$\begin{matrix}{\tau_{c} = \frac{\lambda_{D}a}{D}} & (1)\end{matrix}$

for a conductive cylinder in an electric field, where λ_(D) is the Debyelength, α is the particle radius, and D the diffusion constant of thecharge carrier in the mobile phase. Although pigment particles aretypically composed of dielectric materials, electrical conduction withinthe Stern layer may occur through a number of known mechanisms,including proton hopping (particularly in the presence of adsorbedwater) and therefore the assumption of conductivity is not unreasonablein the present context. The following discussion is included in order toprovide a qualitative, heuristic picture of some of the mechanisms thatmay occur in the present displays and does not in any way limit thescope of the invention.

Equation (1) above shows that the charging time gets longer as theparticle gets larger. The maximum induced-charge electro-osmoticvelocity is then estimated as:

$\begin{matrix}{{{U_{\max} = \frac{\text{?}}{\text{?}}}{\text{?}\text{indicates text missing or illegible when filed}}}\mspace{275mu}} & (2)\end{matrix}$

where E is the magnitude of the applied field, ω the angular frequencyand η the viscosity of the mobile phase. It can be seen that as thefrequency increases the maximum induced-charge electro-osmotic velocitydecreases. At high frequencies, where ω²τ_(c) ²>>1, the induced-chargeelectro-osmotic velocity becomes very low (because there is no time tocharge up the double layer) and screening of the induced dipole by thecharge carriers in the fluid is reduced. At such frequenciesparticle-particle interactions leading to chaining, or interactions withfield gradients leading to dielectrophoretic mobility, may occur. Thus,applying an AC field with increasing frequency may at low frequenciescause electro-osmotic flow, as described above, but as the frequency isincreased particle-particle interactions and dielectrophoresis maydominate. The result may be, for example, that all the particlesconcentrate by induced-dipole aggregation. They are most likely toconcentrate in regions where, at the electro-osmotic driving frequency,stationary points of the flow may be located (i.e., in a ring around theequator in the case of particles within a spherical capsule). Thefrequency at which particle concentration occurs (in the idealized case)is proportional to the applied field and also depends inversely upon theparticle size, since:

$\begin{matrix}{{{U_{\max} = \frac{\text{?}}{\text{?}}}\mspace{20mu} {\text{?}\text{indicates text missing or illegible when filed}}}\mspace{326mu}} & (3)\end{matrix}$

The present inventors have observed that when a display is addressed atrelatively low AC frequencies (typically in the range of 30-100 Hz)encapsulated pigment particles exhibit high-speed motion and becomeuniformly distributed in a capsule. At higher frequencies the pigmentparticles' motion slows down and they may collect at the capsule walls,leading to transparency (shuttering). If the frequency is suddenlyincreased from a relatively low to a relatively high value, however,there may not be time for the particles to reach the stationary pointsof flow at the wall, and they will become immobilized but not shuttered.Thus, for optimal shuttering it is preferred that either the frequencybe ramped from a low to a high value at constant voltage, or that thevoltage be ramped from a high to a low value at constant frequency, orsome combination of these two ramps.

The frequency at which the transition from ICEO motion to a stationarystate occurs is related to (and in some cases proportional to) theapplied voltage and depends upon particle properties such as size, shapeand conductivity and to fluid properties such as viscosity, conductivityand dielectric constant. Thus, pigments may be selectively andindependently addressed by varying the frequency of AC used to drive adisplay. For example, a first pigment in a first capsule may have alarge size and be incorporated into a fluid of high viscosity, whereas asecond pigment in a second capsule layer may have a small size and beincorporated into a fluid of low viscosity. At a lowest addressingfrequency ω₁, both pigments will be distributed and spread out byelectro-osmotic motion. At a higher addressing frequency ω₂ the firstpigment may be shuttered while the second pigment is still in motion. Ata yet higher frequency ω₃ both pigments may be shuttered. If thefrequency is suddenly switched from ω₁ to ω₃ (or to a frequency slightlylower than ω₃ and then increased to ω₃) there may not be time for thefirst pigment to shutter but there may be time for the second pigment todo so. Thus, using a constant voltage and varying only the frequency ofaddressing, it is possible to access different colors while driving eachcolor with the same waveform. It will be clear that the same effect maybe obtained by holding the frequency constant and varying the voltage,and that such a scheme may be extended to more than two colors. Examples1-3 below describe addressing more than one color in a single capsulelayer using alternating voltages of different frequencies. (It will ofcourse be apparent that, when the frequency at which the transition fromICEO motion to a stationary state occurs for different particles iscontrolled by particle properties alone it is possible to incorporateparticles of more than one color into a common capsule rather thanproviding a separate environment for each pigment, as described above.)

Another method for using shuttering a plurality of pigments whiledriving all pigments with a common waveform is illustrated in FIGS.8A-8D, which show in schematic cross-section a two layer display of thepresent invention having a first layer of microcapsules containing afirst pigment coated onto a substrate bearing a concentrator electrodesuch as a grid electrode, over which is coated a second layer ofcapsules containing two oppositely-charged pigments, one of which is(the second pigment) is colored and the other of which is white. (Itshould be noted that the display shown in FIGS. 8A-8D is intended to beviewed from below as illustrated.) Addressing of the two colors (to thefour possible extreme states of color 1, color 2, color 1 plus color 2,and no color) is achieved by taking advantage of differences between thetiming of shuttering in the first layer and the timing of verticalswitching in the second layer. FIGS. 8A-8D illustrate four differentstates of the display. In FIG. 8A, the first pigment 802 is shuttered tothe concentrator electrodes 804 and the second pigment 806 is visible infront of the white pigment 808. In FIG. 8B, the pigment 802 is spreaduniformly across the first layer, while the second layer is in the samestate as in FIG. 8A, so that the display shows the first and secondpigments 802 and 806 together against a white background. In FIG. 8C,the second pigment 806 is hidden behind the white pigment 808 and thefirst pigment is shuttered by being concentrated at the uppermostvertices of the upper pyramidal sections of the first layer of capsules,so that the display shows the white pigment. In FIG. 8D, the firstpigment 802 is spread uniformly over the upper surfaces of the firstlayer of capsules, while the second layer is in the same state as inFIG. 8C, so that the display shows the first pigment against on a whitebackground. A display of this kind is described in detail in Example 4,below.

The necessary independent control of the two capsule layers in FIGS.8A-8D may be achieved by exploiting differences in the rate of switchingbetween the two types of capsule. Such differences in rate can benon-linear with applied voltage if, for example, the fluid within acapsule exhibits a yield stress, such that with an applied voltage belowa threshold value no pigment motion occurs within the capsule. Thus, forexample, if the first layer of capsules shown in FIGS. 8A-8D above havea voltage threshold for switching of V1 and the second layer of capsuleshave a voltage threshold of V2, where V2>V1, the second layer could beaddressed at a voltage greater than V2 (which will also switch the firstlayer) after which the first layer could be switched at a voltage V1between V1 and V2 (without affecting the second layer). Such a schemecan be extended to three layers of capsules having three voltagethresholds.

The necessary voltage thresholds may be provided in a variety of ways.As mentioned above, the internal phase of a microcapsule may have ayield stress. Particles of opposite charge to the shuttering pigment maybe added to the capsule to create a Coulombic threshold. These particlesof opposite charge may be substantially non-scattering andnon-absorbing, such that they do not affect the absorption of light bythe colored pigment and do not affect the overall appearance of thedisplay, being incorporated simply to modulate the switching behavior ofthe interior of the capsule.

Another technique to address capsule layers independently is to make onelayer of capsules multi-stable (i.e., to endow this layer with imagehysteresis, as may be achieved by providing a polymer dispersed in thefluid—see U.S. Pat. No. 7,170,670, and to make a second layer that doesnot exhibit hysteresis and reverts to a default state when not activelyaddressed. FIGS. 9A and 9B illustrate the color changes of such adisplay in terms of their projection on the a*/b* plane of the La*b*color space. Arrow i in FIG. 9A represents switching from cyan to white,such as could occur in the first layer of capsules in FIGS. 8A-8D. Whena field of either polarity is applied, the pigment shutters from adispersed state, and, if it is assumed that the image so formed is notstable, when the field is removed the shuttered pigment redisperses andthe default cyan state is re-formed. Arrow (ii) shows the switching ofwhite to yellow, such as could occur in the second layer of capsulesshown in FIGS. 8A-8D. This switching is hysteretic, such that the finalstate is stable until the field direction is reversed.

FIG. 9B illustrates how four colored states can be achieved in thisdisplay when the switching speeds of capsules in the first and secondlayers are the same. Arrow (1) shows the display being driven byapplication of an electric field such that the first layer of capsulesis driven to white (from the default cyan state) and the second layer isdriven from white to yellow. Arrow (2) shows that as the driving fieldis reduced to zero the second layer remains in the yellow state but thefirst layer defaults to cyan, resulting in the combination of yellow andcyan, which is green. Arrow (3) shows that as the display is then drivenwith an electric field of the opposite polarity the first layer is againdriven to white, while the second layer is driven from yellow to white.The result is a white image. Finally, arrow (4) shows that as the fieldis reduced to zero from the reversed polarity drive, the second layerremains white while the first layer reverts to cyan, resulting in a cyanimage. Different levels of the colors may be attained by changing thetime of driving at a particular voltage (pulse-width modulation) or bychanging the drive voltage.

A larger color gamut may be achieved if the rates of switching of thetwo layers are not the same. FIGS. 10A and 10B are projections similarto FIGS. 9A and 9B respectively but show the color changes occurringwhen the cyan shuttering layer is faster than the white/yellow layer butnot so fast that cyan can be completely shuttered without any switchingof the yellow layer. It is assumed that the white/yellow layer exhibitshysteretic image stability whereas the cyan shutter does not (althoughsimilar principles apply if both layers exhibit hysteretic imagestability). In FIG. 10A there is seen the path that is followed as thewhite layer is switched to yellow. At short times of switching the cyanlayer is completely shuttered whereas the white layer has not completelyswitched to yellow. This provides a mid-yellow state shown as x in FIG.10A. Relaxation of the cyan shutter provides a blue-green color shown asy. Continued switching of the yellow provides a completely yellow state,from which green is obtained by relaxation of the cyan shutter. As isshown in FIG. 10B, the path followed when switching from yellow to whiteis not the same, although it visits the same points at the extremes(i.e., open and closed) of the cyan shutter. It is noteworthy that thepure yellow and white states are only attained with one direction ofdriving. If the cyan shutter does not exhibit hysteretic imagestability, a holding voltage is required to maintain a state which isnot the default.

FIG. 11 shows a schematic side elevation of a three-layer display of thepresent invention. This three-layer display is generally similar to thetwo-layer display shown in FIGS. 8A-8D, except that the concentratorelectrodes are replaced by a first layer of capsules whose walls aremore conductive than their interior. This layer of capsules may containno pigment and be transparent, serving simply to act as a template andshape the second layer of capsules (containing the first pigment) andprovide a capsule wall shutter. The mechanism by which the display ofFIG. 11 achieves the four possible extreme states of two primary colorsis exactly analogous to that described above with reference to FIGS.8A-8D. A three-layer display of this kind is described in detail inExample 5 below.

A third primary color may be provided in the displays shown in FIGS.8A-8D and 11 in several different ways. FIG. 12A illustrates provisionof a third primary color that is shuttered using any of the threemethods mentioned above (a concentrator electrode is shown, but capsulewall/geometrical shuttering or side-wall shuttering may be used also).

FIG. 12B illustrates provision of a third color in the same layer as thesecond color, i.e., in combination with a white pigment, and switchedvertically. Again, the third color should be switched considerably moreslowly than either the first or the second colors. In the display ofFIG. 12B, the second and third colors are arranged side-by-side, whichis a less favorable configuration that shown in FIG. 12A. It ispossible, however, for the capsules containing the third color to be ofa different size from those containing the second color, and thattherefore some overlap of the capsules may be achieved. In addition,some mixing of the second and third colors may be achieved by lightscattering within the structure. FIG. 12B shows a capsulewall/geometrical shutter for the first pigment, but a layer of capsulesmay be eliminated and concentrator electrodes used instead to shutterthe first pigment.

FIGS. 12C and 12D show two different optical states of a three-layerdisplay of the present invention generally similar to the displays shownin FIGS. 12A and 12B. In the display shown in FIGS. 12C and 12D, thefirst electro-optic layer (the lowest layer as illustrated in FIGS. 12Cand 12D) contains a yellow pigment, is state-stable, has a threshold andrequires a high operating voltage. The yellow pigment in this layer ismovable between the shuttered state shown in FIG. 12C, in which thepigment is confined to the uppermost vertex of the pyramidal uppersection of each capsule in this layer, and the unshuttered state shownin FIG. 12D, in which the yellow pigment covers the whole of the flatlower surface of each capsule. The second electro-optic layer of thedisplay shown in FIGS. 12C and 12D contains a cyan pigment and is notstate-stable. The cyan pigment in this layer can be moved between thetwo shuttered positions shown in FIGS. 12C and 12D, in which the pigmentis confined to the uppermost and lowermost vertices of each capsule, andan unshuttered position (not shown) in which the pigment is distributeduniformly throughout each capsule. The third electro-optic layer of thedisplay contains a magenta pigment which can be moved vertically (asillustrated) between the unshuttered position shown in FIG. 12C and theshuttered position shown in FIG. 12D. The upper electrode shown in FIGS.12C and 12D is provided with a white reflector.

As already indicated, in one aspect the present invention provides avariable transmission electrophoretic medium comprising a plurality ofdifferent organic pigments in a fluid, such that when all the pigmentsare dispersed substantially uniformly through the fluid, the mediumappears substantially black. Also, the medium is substantially free frompolymeric additives in the fluid (other than charge control or similaragents, and surfactants) but still has a high degree of image stability.In a preferred form of such a variable transmission medium, the pigmentsare chosen to have substantial different dielectrophoretic mobilities,so that by manipulating the frequency and voltage applied to theelectrophoretic medium, each pigment individually can be made to be in adispersed state or in a packed state, and the medium can display asubstantial color gamut.

As is well known to those skilled in the pigment art, a blend of two ormore color pigments can be made to provide additional colors, includingblack. A broad range of color pigments has been developed for demandingapplications such as automotive paints, and some of these pigments havedemonstrated suitable properties for VT media. Pigments of thequinacridone and phthalocyanine families have been found useful. Suchpigments can be blended to yield in total a very broad color gamut, butthe added constraint of requiring black to be part of the gamut hashitherto diminished the color gamut achievable in a single medium.

Somewhat unexpectedly, it has been found that that some of the pigmentsuseful in VT media have been found to display a wide range ofelectrophoretic mobilities in the sense of differing responses to a widerange of applied frequencies and voltages. Accordingly, by manipulatingthe frequency and voltage applied to the electrophoretic medium, eachpigment individually can be made to be in a dispersed state or in apacked state, i.e., the different organic pigments can be made to“shutter” independently of one another. The dispersed state allows thepigment to absorb the transmitted light, the more uniform thedispersion, the better the absorption. The packed state minimizes theareal fraction of the medium where the pigment is located, therebyminimizing the absorption by that pigment. If all the pigments arepacked, then the medium will assume its “open” or substantiallytransparent optical state. If, on the other hand, all the pigments aredispersed throughout the fluid, the medium will assume a substantiallyblack optical state, provided with amounts and colors of the variouspigments are balanced to achieve a neutral color. If at least onepigment is dispersed and at least one pigment is packed, the color ofthe medium will approach the color of the dispersed pigment, and byindependent control of the state of dispersion of the various pigments,a substantial gamut of colors can be produced; for reasons which will befamiliar to those skilled in the color imaging art, it is normallypreferred that such a VT medium contain at least three differentpigments having differing colors and dielectrophoretic mobilities.

The displays of the present invention can, as already discussed, makeuse of various driving methods to drive three separate pigments (whetherthose pigments be present in one, two or three separate electro-opticlayers). Perhaps surprisingly, it is possible to drive three separatepigments substantially independently of one another using only a singleset of electrodes and DC voltages. Conceptually, the methods for drivingthree different pigments using only DC and a single set of electrodes,may be summarized as follows:

Assume that there are three color-forming layers (although the principledoes not require layers) with the following properties:

-   -   (a) Layer 1 has a voltage (or impulse) threshold, is state        stable, and its color depends upon the polarity of the applied        voltage;    -   (b) Layer 2 has either a lower threshold or no threshold, is        state stable, and its color depends upon the polarity of the        applied voltage; and    -   (c) Layer 3 has no threshold, is not state stable, and switches        faster than layer 2. Layer 3 reaches the same state whether        driven with a positive or a negative impulse and relaxes to its        opposite extreme when no potential is applied.

Given these three conditions, the drive scheme is:

-   -   (a) Use a high voltage to set layer 1 to the desired color. In        practice, this could be binary and in this case should be        yellow. This will also affect layers 2 and 3.    -   (b) Use a lower voltage to set layer 2 to the desired color.        This will not affect layer 1, and will switch layer 3 to its        extreme state.    -   (c) Allow layer 3 to relax to its desired color and keep it        there with a holding voltage (or pulses at any voltage).

The alternative to this, if all three layers are state stable, is themore straightforward scheme of addressing sequentially at threedifferent voltages for the three colors and correcting the collateralswitching of the faster layers when addressing the slower ones. In thiscase the slowest layer is addressed first, and layers 1 and 2 each havea threshold. However, this is much harder to engineer.

The following Examples are now given, though by way of illustrationonly, to show details of preferred reagents, conditions and techniquesused in the media of the present invention.

Example 1 First Medium Containing Red, Green and Blue Pigments

The pigments used in this Example were:

-   -   Clariant Hostaperm Pink E 02, a red quinacridone pigment        (available commercially from Clariant Corporation, 4000 Monroe        Road, Charlotte N.C. 28205), stated by the manufacturer to have        a specific gravity of 1.45, a surface area of 77 m²/g, and an        average particle size of 90 nm;        -   Clariant Hostaperm Green GNX, a green copper phthalocyanine            pigment from the same manufacturer, and stated by the            manufacturer to have a specific gravity of 2.05, a surface            area of 40 m²/g, and an average particle size of 50 nm; and        -   Clariant Hostaperm Blue B2G-D, a blue copper phthalocyanine            pigment from the same manufacturer, and stated by the            manufacturer to have a specific gravity of 1.6, a surface            area of 44 m²/g, and an average particle size of 75 nm.

Millbases of each pigment containing approximately 20 percent by weightof the pigment in d-limonene and 0.0044 g of Solsperse 17000 per squaremeter of pigment surface area (as stated by the manufacturer) wereprepared substantially as described in the aforementioned 2007/0146310.Samples of each millbase were diluted to 0.01 weight percent pigmentwith d-limonene and the resultant dispersion measured for colortransmission with a Minolta CM-3600d spectrometer using 2 mm path lengthsample cuvettes. The millbases were also blended to produce a mixeddispersion having a color as close to black as possible (i.e., havingthe smallest possible a* and b* values in the conventional CIE L*a*b*color space). The results are shown in Table 1 below.

TABLE 1 Color (saturated) L* a* b* % R % G % B Red 37.1 55.7 −17.4 100 00 Green 21.1 10.1 −28.5 0 100 0 Blue 22.2 22.6 −17.6 0 0 100 Black 31.00 −17.1 43.2 0 56.8Note that the “black” blend actually tended to green even without addingany green pigment; this was caused by the blue-green hue of the “blue”pigment.

Example 2 Second Medium Containing Red, Green and Blue Pigments

Example 1 was repeated except that Clariant Ink Jet Magenta E02 VP 2621,a quinacridone pigment having an average particle size of 70 nm, wassubstituted for the pink pigment used in Example 1. Again, the millbaseswere also blended to produce a mixed dispersion having a color as closeto black as possible. The results are shown in Table 2 below.

TABLE 2 Color (saturated) L* a* b* % R % G % B Red 39.6 45.6 −17.6 100 00 Green 21.1 10.1 −28.5 0 100 0 Blue 22.2 22.6 −17.6 0 0 100 Black 31.00 −16.9 47.4 0 52.6Again, the blended “black” tended to green, and since a suitablealternative blue pigment could not be located, it was determined that amajor change in the pigment set was necessary.

Example 3 Medium Containing Green, Violet and Yellow Pigments

Example 1 was repeated except that the pink and blue pigments werereplaced with Clariant Hostaperm Violet RL02 and Clariant NovopermYellow 4G VP2532 from the same manufacturer. The former is a dioxazinepigment stated by the manufacturer to have a specific gravity of 1.49, asurface area of 80 m²/g, and an average particle size of 50 nm, whilethe latter is a disazo pigment stated by the manufacturer to have aspecific gravity of 1.44, a surface area of 33 m²/g, and an averageparticle size of 162 nm. Again, the millbases were also blended toproduce a mixed dispersion having a color as close to black as possible.The results are shown in Table 3 below.

TABLE 3 Color (saturated) L* a* b* % G % P % Y Green 21.1 10.1 −28.5 1000 0 Purple 30.4 4.2 20.8 0 100 0 Yellow 46.2 −2.7 85.2 0 0 100 Black31.0 0 0 57.8 22.9 19.3As may be seen from Table 3, the blend of this set of pigments producesa good, neutral black.

A polymer-dispersed electrophoretic medium was produced using this blackblend in substantially the same manner as described in Example 1 of U.S.Pat. No. 6,866,760; the polymer-dispersed medium was coated on theindium-tin-oxide (ITO) coated surface of a polyethyleneterephthalate/ITO film, dried, and adhesive layer applied, and theresultant film laminated to a rear electrode to produce an experimentalsingle-pixel display, which were then driven at various combinations ofdrive voltages and frequencies.

In such a multi-pigment display, if all the pigments are packed, thedisplay appears open (substantially transparent); if all the pigmentsare dispersed the display appears closed (substantially black). When thefractions of each pigment in its dispersed form is different from theoverall proportion of that pigment in the display (i.e., different fromthe 57.8% green, 22.9% violet, 19.3% yellow shown in Table 3), then thedisplay color approaches the color of the more dispersed pigment. Forexample, if the yellow pigment is well dispersed, and the green andpurple pigments are packed, the display will appear yellow.Alternatively, if the yellow and green pigments are dispersed and onlythe purple is packed, the display will appear yellow-green. Table 4below gives examples of applied waveforms and the corresponding colorsproduced.

TABLE 4 Color L* a* b* Waveform OPEN 53.0 −4.2 4.9 120 V, 5 kHz sinewave CLOSED 24.7 −3.3 −2.5 120 V, 60 Hz sine wave GREEN 42.2 −25.9 8.2120 V, 0.5 Hz square wave YELLOW 47.0 −5.8 16.7 Open, then 10 ms burstsof 2 kHz sine wave, 0.5 seconds apart BLUE 26.6 0.2 −13.3 120 V, 1.2 kHzsine waveThe above colors, and others produced using different waveforms, areplotted in the a*b* plane in FIG. 17 of the accompanying drawings. Thecolors were measured with an Eye-One spectrophotometer in reflectivemode with a white background placed behind the sample. The whitebackground was treated as the reference white-point for the L*a*b*calculations.

It will be seem from the Figure that the experimental display wascapable of displaying a substantial color gamut, though only in thegreen/yellow/blue portion of the a*b* plane; this particular display wasnot capable of producing a positive a* (i.e., a red color). However, inview of the substantial color gamut capable of being displayed by thisexperimental display, and the wide range of pigments commerciallyavailable and suitable for incorporation in such a display it can beanticipated that evaluation of additional pigments and further work informulation will produce displays have a broader color gamut morecentered in the a*b* plane.

Example 4 Coating Slurry for Cyan Shuttering Capsules

A cyan pigment, Irgalite Blue GLVO (available from BASF, Ludwigshafen,Germany) (8 g) was combined with Isopar E (12 g) and a solution ofSolsperse 17000 (available from Lubrizol Corporation, Wickliffe, Ohio,20 g of a 20% w/w solution in Isopar E) and the mixture was dispersed bystirring with beads to afford a cyan pigment dispersion.

The cyan pigment dispersion thus prepared (5.75 g) was combined withIsopar E (109.25 g) and the resultant mixture mechanically rolledovernight to produce an internal phase ready for encapsulation. Theinternal phase so prepared was then encapsulated following the proceduredescribed in U.S. Pat. No. 7,002,728. The resultant encapsulatedmaterial was isolated by sedimentation, washed with deionized water, andsize separated by sieving, using sieves of 45 and 20 μm mesh. Analysisusing a Coulter Multisizer showed that the resulting capsules had a meansize of 40 μm and more than 85 percent of the total capsule volume wasin capsules having the desired size of between 20 and 60 μm.

The resulting capsule slurry was adjusted to pH 9 and excess waterremoved. The capsules were then concentrated and the supernatant liquiddiscarded. The capsules were mixed with an aqueous polyurethane binder(prepared in a manner similar to that described in U.S. PatentApplication No. 2005/0124751) and small amounts of Triton X-100surfactant and hydroxypropylmethyl cellulose were added and mixedthoroughly to provide a slurry ready for use in a display, as describedin detail below.

Example 5 Coating Slurry for Cyan Shuttering Capsules Containing 1% w/wof Pigment

Irgalite Blue GLVO (26 g) was combined with Isopar E (70 g) and asolution of Solsperse 17000 (70 g of a 20% w/w solution in Isopar E) andthe mixture was dispersed in an attritor with glass beads to produce acyan pigment dispersion. The cyan pigment dispersion thus prepared (5.75g) was combined with Isopar E (109.25 g). The resultant mixture wasmechanically rolled overnight to produce an internal phase ready forencapsulation. The internal phase so prepared was then encapsulatedfollowing the procedure of Example 4 above to produce capsules having amean size of 40 μm and with more than 85 percent of the total capsulevolume in capsules having the desired size of between 20 and 60 μm. Thecapsules were then incorporated into a coating slurry in the same way asin Example 4 above.

Example 6 Coating Slurry for Magenta Shuttering Capsules

A magenta pigment, Quindo Red 19 (available from Sun ChemicalCorporation, Parsippany, N.Y.) was provided with a poly(laurylmethacrylate) coating substantially as described in U.S. Pat. No.7,002,728. The coated pigment (13 g) was then combined with Isopar E (30g) to produce a magenta pigment dispersion, which was filtered through a200 μm mesh film and the percent solids determined to be 17%.

The magenta pigment dispersion thus prepared (13 g) was combined withIsopar E (88 g) and Solsperse 17000 (8 g of a 20% w/w solution in IsoparE), and the resultant mixture was rolled mechanically overnight toproduce an internal phase ready for encapsulation. The internal phase soprepared was then encapsulated as described in Example 4 above toproduce capsules having a mean size of 40 μm and with more than 85percent of the total capsule volume in capsules having the desired sizeof between 20 and 60 μm. The capsules were then incorporated into acoating slurry in the same way as in Example 4 above.

Example 7 Coating Slurry for Magenta/White Vertically Switching Capsules

Functionalized magenta pigment (10 g, prepared as described in Example 6above) was combined with Isopar E (40 g) and the resultant mixturedispersed by stirring with beads to produce a magenta pigmentdispersion, which was filtered through a 200 μm mesh film and thepercent solid determined. This dispersion (18.82 g) was combined withtitanium dioxide (R794 available from E. I. du Pont de NemoursCorporation, Wilmington, Del.) (70.57 g of a 60% w/w dispersion treatedas described in U.S. Pat. No. 7,002,728), minor amounts of Solsperse17000 and poly(isobutylene) of molecular weight 600,000, and additionalamounts of Isopar E. The resultant mixture was rolled mechanicallyovernight to produce an internal phase ready for encapsulation. Theinternal phase so prepared was then encapsulated as described in Example4 above to produce capsules having a mean size of 40 μm and with morethan 85 percent of the total capsule volume in capsules having thedesired size of between 20 and 60 μm. The capsules were thenincorporated into a coating slurry in the same way as in Example 4above.

Example 8 Coating Slurry for Yellow/White Vertically Switching Capsules

A yellow pigment, Paliotan Yellow L 1145 (available from BASF), wassurface-treated withN-[3-(trimethoxysilyl)propyl]-N′-(4-vinylbenzyl)ethylenediaminehydrochloride, followed by provision of a poly(lauryl methacrylate)coating substantially as described in U.S. Pat. No. 7,002,728. Thisyellow pigment (30 g) was combined with Isopar E (70 g) and sonicatedfor 2 hours and mechanically rolled overnight. The resultant dispersion(45.6 g) was then combined with the same R794 titanium dioxide as inExample 7 (102.6 g of a 60% w/w dispersion) minor amounts of Solsperse17000 and poly(isobutylene) of molecular weight 850,000, and additionalamounts of Isopar E. The resultant mixture was rolled mechanicallyovernight to produce an internal phase ready for encapsulation. Theinternal phase so prepared was then encapsulated as described in Example4 above to produce capsules having a mean size of 40 μm and with morethan 85 percent of the total capsule volume in capsules having thedesired size of between 20 and 60 μm. The capsules were thenincorporated into a coating slurry in the same way as in Example 4above.

Example 9 Coating Slurry for Capsules Containing No Pigment

A solution of Solsperse 17000 in Isopar E) was rolled mechanicallyovernight to produce an internal phase ready for encapsulation. Theinternal phase so prepared was then encapsulated as described in Example4 above to produce capsules having a mean size of 40 μm and with morethan 85 percent of the total capsule volume in capsules having thedesired size of between 20 and 60 μm. The capsules were thenincorporated into a coating slurry in the same way as in Example 4above.

Example 10 Assembly and Switching of a Cyan/Yellow Bichrome Display

The cyan shuttering capsule slurry prepared in Example 4 above wascoated on to a 125 μm poly(ethylene terephthalate) (PET) film bearing aprinted hexagonal grid metallic electrode of 50 μm pitch and 1 μm linewidth using a bar coater with a 50 μm gap. The coating was dried at 60°C., following which a second coating of capsules was applied using theyellow/white vertical switching capsule slurry prepared as described inExample 8 above, with a bar coater gap of 100 μm. The second coatinglayer was dried at 60° C. A layer of polyurethane adhesive doped with atetraalkylammonium salt, pre-coated onto a release sheet, was laminatedon top of the second layer of capsules as described in U.S. Pat. No.7,002,728. The release sheet was removed and the resultant multilayerstructure was laminated onto a graphite rear electrode. As finallyassembled the display structure comprised, in order from its viewingsurface, a first layer of PET film base, a second layer of patternedelectrode, a third layer of cyan shuttering capsules, a fourth layer ofyellow/white vertically switching capsules, a fifth layer of aconductively doped lamination adhesive, and a sixth layer comprising agraphite rear electrode.

The display structure so produced was driven by applying a square waveAC signal at 30 Hz and +/−10 V that was offset by DC voltages thatranged from 40 V to −40 V. As the display was driven it was illuminatedby a tungsten ring-light source and light reflected from the display wasanalyzed spectrophotometrically, giving the L*a*b* values shown in Table5 below.

TABLE 5 Time Offset (s) Voltage L* a* b* 1 40 47.83 −22.68 −22.03 2 3049.11 −21.02 −20.92 3 20 49.24 −21.07 −21.20 4 10 48.53 −22.38 −22.52 50 41.32 −37.16 −33.34 6 −10 41.67 −35.22 −7.91 7 −20 44.86 −29.27 6.13 8−30 46.81 −27.42 8.74 9 −40 47.86 −25.45 9.46 10 −30 48.27 −24.84 10.1411 −20 48.29 −24.97 10.16 12 −10 47.42 −26.88 9.60 13 0 41.09 −42.042.31 14 10 41.84 −36.48 −15.86 15 20 46.37 −25.72 −21.39 16 30 48.33−22.44 −21.64 17 40 49.11 −21.16 −20.17 18 30 49.31 −20.89 −19.84 19 2049.26 −21.10 −20.09 20 10 48.60 −22.24 −21.40 21 0 41.77 −36.03 −31.51

These results are shown graphically in FIGS. 13A and 13B. FIG. 13A showsthe L*, a* and b* values as a function of the DC offset applied to the30 Hz AC drive at the top-plane, transparent electrode. Note thatsimilar states are obtained with a simple DC drive, without the 30 Hz ACcomponent, although the resultant states are slightly inferior.

This display operates as shown in FIGS. 8A-8D. The first layer ofcapsules, in contact with the concentrator electrodes on the top-planeside and with the second layer of capsules on the other, contains apositively-charged cyan pigment. When the DC offset is positive, thecyan pigment is shuttered at the junction between the first layer ofcapsules and the second layer of capsules (as shown in FIG. 8C). Whenthe DC offset is negative, the cyan pigment collects at the concentratorelectrodes (as shown in FIG. 8A). As the DC offset moves towards zerothe pigment moves away from the shuttered state. The hysteresis loopseen in the a* value in FIG. 13A reflects these changes (less negativea* corresponds to the shuttered cyan pigment). There is a slighthysteresis, as the movement of pigment towards the shuttered state asthe DC offset is increased is faster than the movement away from theshuttered state as the DC offset is decreased.

The vertically-switching capsules (i.e., capsules in which the particlesmove parallel to the applied electric field) in the second capsule layercontain a positively-charged yellow pigment and a negatively-chargedwhite pigment, and thus when the top-plane is negatively charged thislayer of capsules displays a yellow image (positive b*) and when thetop-plane is positively charged a white image (negative b* because ofthe blue component in the overlying cyan layer).

The a*/b* plot shown in FIG. 13B corresponds experimentally to thatshown conceptually in FIG. 9B.

Example 11 Assembly and Driving of a Cyan/Magenta Bichrome Display inwhich a Layer of Pigment-Less Capsules is Used to Direct Shuttering

The display produced in this Example has the structure shown in FIG.12A.

The pigment-less capsule slurry prepared in Example 9 above was coatedon to a PET film having a transparent, conductive coating of indium tinoxide (ITO) and dried using the same conditions as the first layer ofcapsules in Example 10 above. A second coating of capsules was appliedusing the cyan shuttering capsule slurry prepared in Example 5, above,with a bar coater gap of 80 μm, and the coating was dried at 60° C. Athird coating of capsules was applied using the magenta/white verticalswitching capsule slurry prepared in Example 7 above using a bar coatergap of 100 μm to form a third capsule layer, which was dried at 60° C.An adhesive layer was laminated on top of the third layer of capsules inthe same manner as in Example 10 above. The release sheet was removedand the resultant multilayer structure was laminated onto a graphiterear electrode. As finally assembled the display structure comprised, inorder from its viewing surface, a first layer of PET film base, a secondlayer of an unpatterned, continuous transparent electrode, a third layerof pigment-less capsules, a fourth layer of cyan shuttering capsules, afifth layer of magenta/white vertically switching capsules, a sixthlayer of adhesive, and a seventh layer comprising the graphite rearelectrode.

The display structure so produced was driven by applying a square waveAC signal at 30 Hz and +/−10 V that was offset by DC voltages thatranged from 40 V to −40 V. As the display was driven it was illuminatedby a tungsten ring-light source and light reflected from the display wasanalyzed spectrophotometrically, giving the L*a*b* values shown in Table6 below.

TABLE 6 Time Offset (s) Voltage L* a* b* 1 40 47.55 −29.33 −41.46 2 3049.64 −25.95 −38.86 3 20 49.35 −25.66 −38.95 4 10 48.27 −26.97 −40.10 50 44.22 −34.81 −46.48 6 −10 39.55 −30.70 −45.24 7 −20 31.31 −5.77 −37.808 −30 31.21 4.53 −33.22 9 −40 32.80 5.97 −31.11 10 −30 32.88 5.63 −30.9911 −20 32.32 4.83 −31.93 12 −10 31.22 3.01 −33.67 13 0 28.32 −2.14−37.97 14 10 29.82 −6.63 −38.78 15 20 41.69 −20.62 −40.66 16 30 48.19−23.62 −39.06 17 40 50.44 −23.52 −37.55 18 30 50.43 −23.76 −37.58 19 2049.76 −24.34 −38.09 20 10 48.60 −25.70 −39.86 21 0 44.52 −33.51 −45.99

These results are shown graphically in FIGS. 14A and 14B. FIG. 14A showsthe L*, a* and b* values as a function of the DC offset applied to the30 Hz AC drive at the top-plane, transparent electrode. Note that, aswas the case in Example 10 above, similar states are obtained with asimple DC drive, without the 30 Hz AC component, although the resultantstates are slightly inferior.

The display operates as follows. The second layer of capsules, incontact with the pigment-less capsules on one side and with the thirdlayer of capsules on the other, contains a positively-charged cyanpigment and shutters as shown conceptually in FIG. 4. When the DC offsetis positive, the cyan pigment is shuttered at the junction between thesecond layer of capsules and the third layer of capsules. When the DCoffset is negative, the cyan pigment is shuttered at the junctionbetween the second layer of capsules and the first layer of(pigment-less) capsules. The shuttering of the cyan pigment is mosteasily seen in the a*/b* plot shown in FIG. 14B, in which arrow icorresponds to the movement between shuttered and unshuttered cyanpigment when the top-plane is positively charged and arrow iicorresponds to the movement between shuttered and unshuttered cyanpigment when the top plane is negatively charged.

The vertically-switching capsules contain a positively-charged magentapigment and a negatively-charged white pigment, and thus when the topplane is negatively charged this layer of capsules displays a magentaimage (more positive a*) and when positive a white image (more negativea*). This switching corresponds to arrow iii in FIG. 14B.

Example 12 Cyan/Magenta/Yellow Trichrome Display as Per FIG. 12A

The first two coating steps of Example 10 above were repeated using themagenta shuttering capsule slurry prepared in Example 6 above for thefirst coated layer and the cyan shuttering capsule slurry prepared inExample 5 above for the second coated layer, except that in the secondcoating the bar coater gap was 80 μm. Next, the yellow/white verticalswitching capsule slurry prepared in Example 8 above was applied with abar coater gap of 100 μm to form a third capsule layer, which was driedat 60° C. An adhesive layer was laminated on top of the third layer ofcapsules, the release sheet removed and the remaining layers laminatedon to a graphite rear electrode, all as in Example 10. As finallyassembled the display structure comprised, in order from the viewingsurface, a first layer of PET film base, a second layer of patternedelectrode, a third layer of magenta shuttering capsules, a fourth layerof cyan shuttering capsules, a fifth layer of yellow/white verticallyswitching capsules, a sixth layer of a conductively doped laminationadhesive, and a seventh layer comprising a graphite rear electrode.

The display structure was driven, illuminated and the light reflectedtherefrom analyzed, all in the same manner as in Example 10. The resultsare shown in Table 7 below.

TABLE 7 Time Offset (s) Voltage L* a* b* 1 40 47.48 16.47 −4.23 2 2048.75 15.92 −3.60 3 10 48.15 16.30 −3.88 4 5 46.92 16.95 −4.92 5 0 42.2517.44 −9.20 6 −5 39.03 12.09 −8.61 7 −10 39.68 11.03 8.37 8 −20 42.359.32 19.65 9 −40 46.18 6.57 24.16 10 −20 46.33 6.44 24.11 11 −10 45.277.52 23.62 12 −5 43.35 9.36 21.91 13 0 39.31 10.41 18.79 14 5 37.75 8.0316.43 15 10 42.03 11.98 13.69 16 20 46.61 14.71 5.12 17 40 49.76 13.57−0.67 18 20 50.09 13.55 −1.86 19 10 49.37 14.05 −2.72 20 5 47.99 14.97−3.76 21 0 43.20 16.23 −7.40These results are shown graphically in FIGS. 15A and 15B which aredirectly comparable to the FIGS. 13A and 13B respectively. It can beseen that a wide range of colors is addressable by the display. Thecolors are offset along the a* axis, indicating that the magenta shutteris not fully closing. Such an offset may be corrected by applying auniform color filter to the entire display.

Example 13 Bichrome Display Using a Single Applied Voltage and TimeModulation

A display was prepared essentially as described in Example 11 above,except that the ITO coating on the PET was pretreated with a compositioncomprising an ionic dopant prior to application of the pigment-lesscapsule layer. This display was addressed by using the waveform shown inTable 8 below.

TABLE 8 FIG. Start End Voltage 16A time time (V) ref. 0 2 −30 a 2 4 0 b4 6 30 c 6 8 0 d 8 8.2 −30 e 8.2 10.2 0 fThe drive continued with pulses at 30V of 200 ms length followed byrests of 2 seconds in length. The colors produced by the display areshown in FIG. 16, in which the letters correspond to those in the lastcolumn of Table 8. A negative voltage shutters the cyan pigment andswitches the magenta/white capsules to magenta, corresponding toposition a in FIG. 16. As the voltage is held at zero, the magenta/whitecapsules, being state-stable, remain in the magenta state but the cyanshutter, not being state-stable, relaxes to the cyan state from theclear state, corresponding to position b in FIG. 16. A positive voltagethen switches the cyan to the shuttered, clear state and the magenta tothe white state, corresponding to position c in FIG. 16. As the voltageis held at zero, the magenta/white capsules, being state-stable, remainin the white state but the cyan shutter, not being state-stable, relaxesto the cyan state from the clear state, corresponding to position d inFIG. 16. Next the polarity of the drive is reversed, but the switchingtime is reduced to 200 ms in pulses separated by 2 second rests. Themagenta/white capsules are incrementally switched from the white to themagenta state (since they are state-stable), whereas the cyan capsulespartially shutter (position e) but relax to the non-shuttered stateevery time the voltage is reduced to zero (position f). This patternrepeats as the short pulses are continued.

From the foregoing discussion, it will be seen that the presentinvention provides variable transmission electrophoretic media capableof displaying multiple colors. When used as a variable color segmentedoverlay, the media of the present invention can provide a much broadercolor gamut than a conventional (static) color filter array. The presentinvention also provides a low haze monochrome variable transmissionmedium with high image stability.

It will be apparent to those skilled in the art that numerous changesand modifications can be made in the specific embodiments of theinvention described above without departing from the scope of theinvention. Accordingly, the whole of the foregoing description is to beinterpreted in an illustrative and not in a limitative sense.

1. An electro-optic display comprising at least first and second layersof electro-optic material, the first layer being capable of displayingat least one optical state which cannot be displayed by the secondlayer, the display further comprising a first electrode disposed on oneside of the first and second layers, and a second electrode disposed onthe opposed side of the first and second layers from the firstelectrode, there being no electrode between the first and second layers.2. An electro-optic display according to claim 1 further comprising athird layer of electro-optic material, the third layer being capable ofdisplaying at least one optical state which cannot be displayed by thefirst and second layers, the second electrode being disposed on theopposed side of the first, second and third layers from the firstelectrode, there being no electrode between the second and third layers.3. An electro-optic display according to claim 2 wherein the threelayers contain cyan, magenta and yellow pigments, in any order.
 4. Anelectro-optic display according to claim 3 wherein the layer containingthe yellow pigment is arranged to have a smaller number of gray levelsthan the layers containing the cyan and magenta pigments.
 5. Anelectro-optic display according to claim 3 having a viewing surfacethrough which an observer views the display, and wherein the twoelectro-optic layers closest to the viewing surface contain cyan andyellow pigments, in either order.
 6. An electro-optic display accordingto claim 1 wherein at least one of the layers of electro-optic materialcomprises an electrophoretic material comprising a plurality of chargedpigment particles dispersed in a fluid and capable of moving through thefluid on application of an electrical field to the layer.
 7. Anelectro-optic display according to claim 6 wherein at least one of thefirst and second layers comprises a microcavity electrophoreticmaterial.
 8. An electro-optic display according to claim 6 wherein thecharged particles in at least one of the layers are movable between afirst optical state, in which the pigment particles occupy substantiallythe whole area of each pixel, and a second optical state, in which thepigment particles occupy only a minor proportion of the area of eachpixel.
 9. An electro-optic display according to claim 8 wherein thecharged particles in each of the first and second layers are movablebetween a first optical state, in which the pigment particles occupysubstantially the whole area of each pixel, and a second optical state,in which the pigment particles occupy only a minor proportion of thearea of each pixel.
 10. An electro-optic display according to claim 9wherein the yield stresses of the fluids in the first and second layersdiffer.
 11. An electro-optic display according to claim 9 wherein thesizes of the microcavities in the first and second layers differ.
 12. Anelectro-optic display according to claim 9 wherein the particles in thefirst and second layers differ in at least one of size, shape andelectrical conductivity.
 13. An electro-optic display according to claim9 wherein the fluids in the first and second layers differ in at leastone of viscosity and electrical conductivity.
 14. An electro-opticdisplay according to claim 9 having a viewing surface at or adjacent thesurface of the first electro-optic layer remote from the secondelectro-optic layer, the electro-optic display further comprising athird electro-optic layer on the opposed side of the secondelectro-optic layer from the viewing surface, the third electro-opticlayer comprising first and second types of particles of differing colorsdisposed in a fluid and capable of moving through the fluid onapplication of an electric field to the third electro-optic layer, thefirst and second types of particles being of differing electrophoreticmobility.
 15. An electro-optic display according to claim 14 wherein thefirst type of particles in the third electro-optic layer are white. 16.An electro-optic display according to claim 15 wherein the particles inthe first and second electro-optic layers are yellow and cyan, in eitherorder, and the second type of particles in the third electro-optic layerare magenta.
 17. An electro-optic display according to claim 1 whereinat least one of the first and second electrodes occupies only a minorproportion of the area of a pixel.
 18. An electro-optic displayaccording to claim 7 wherein the walls of the microcavity have a higherelectrically conductivity than the phase comprising the plurality ofcharged pigment particles dispersed in the fluid.
 19. An electro-opticdisplay according to claim 1 wherein one of the electro-optic layers hasat least two stable states, and the second electro-optic layer has onlyone stable state.
 20. A microcavity electrophoretic display comprisingwalls defining at least one cavity, the cavity containing a fluid andfirst, second and third types of particles dispersed in the fluid, eachof the first, second and third types of particles having an unshutteredstate, in which the particles occupy substantially the entire area ofthe microcavity, and a shuttered state, in which the particles occupyonly a minor proportion of the areas of the microcavity, the first,second and third particles being of differing colors and differing indielectrophoretic and/or electro-osmotic properties such that the first,second and third types of particles can be moved between theirunshuttered and shuttered states independently of one another.
 21. Amicrocavity electrophoretic display according to claim 20 wherein thecolors of the first, second and third types of particles are such thatwhen all three types of particles are in their unshuttered states thedisplay appears substantially black.